(1) Field of the Invention
The invention relates to the transmission of speech signals with reduced bandwidth.
Many different systems for reducing the bandwidth of speech signals are known. These various systems are classified under the common heading vocoder.
The invention particularly relates to voice-excited vocoders. In such a vocoder the low-frequency components of the speech to a frequency which typically is about 800 Hz are not modified but the band of the high-frequency components is subdivided into a plurality of sub-bands and from each sub-band a control signal with a reduced bandwith is derived which is representative of the energy in the relevant sub-band.
Vocoders which comprise a filter unit for analyzing the speech are usually called channel-vocoders. The vocoders discussed in the above paragraph are indicated as channel-vocoders with voice-excitation.
(2) Description of the Prior Art
U.S. Pat. No. 3,030,450 discloses a channel-vocoder with voice-excitation. In the receiver station high-frequency components are reproduced from the non-modified low-frequency band. This band of high-frequency components is used as a common excitation signal which is modulated by the control signals with reduced bandwith for reconstructing the high-frequency parts of the original speech. The reconstructed high-frequency part is then combined with the non-modified low-frequency part for synthetizing a speech signal of a good quality.
According to the U.S. Pat. No. 3,030,450 the arrangement for producing high-frequency components from the non-modified low-frequency band may comprise a non-linear network the output signal of which is applied to a pulse width converter which is controlled by a sawtooth generator. The output signal thereof is applied to a plurality of channel modulators to which also the control signals with reduced bandwidth are supplied. The output signals of the channel modulators are combined with one another by the intermediary of channel filters to a replica of the band of high-frequency components of the original speech.
An alternative arrangement is known from U.S. Pat. No. 3,139,487. This arrangement comprises, in this sequence, a rectifier, a differentiator, a filter unit and an amplitude limiter or a circuit with automatic volume control, connected to each filter of the filter unit. The output signals of the amplitude limiters or the circuits with automatic volume control are applied to the channel modulators to which also the control signals with reduced bandwidth are applied. The output signals of the channel modulator are combined in the same manner as in U.S. Pat. No. 3,030,450.
For a proper operation of these vocoders it is of importance that the frequency spectrum of the excitation signal is flat. The arrangements described above only supply an approximation of a flat spectrum, which approximation also depends on the nature of the unmodified low-frequency part of the speech signal.
Furthermore, for a proper operation of these vocoders it is of importance that the channel modulators have a linear transmission characteristic over a wide dynamic range of, typically, approximately 60 dB.